What are the differences between schizophrenia, alzheimer's and parkinson's disease?

Schiz./Alz./Parkin.: Schizophrenia is a thought disorder with psychotic features. Alzheimer is a cognitive function disorder with dementia. Parkinson is a neurological disorder with tremors/rigidity/slow movements.

Many differences: Ad and pd are "neurodegenerative disease" associated with specific clinical signs for each, loss of selective neurons, and accumulation of "hallmark" lesions (ad - plaques, tangles; pd -lewy bodies) and specific proteins within those leions. Schizophrenia is more chemical than structural/degenerative, due to abnormal Dopamine metabolism, associated signs - delusion, auditory hallucinations, etc.

Psych vs neuro: Schizophrenia is a psychiatric disorder with symptoms of hallucination and paranoia . Parkinson's is more a condition of motor function or movement disorder and not a behavioral disease . Alzheimer's disease is a condition of cognitive loss and in late stages can involve paranoia and psychotic behavior.

Symptoms can overlap: These are 3 different illnesses without known linkages but all effect brain functioning. If you have one of these illnesses you could have symptoms suggestive of the others.

Different diseases: These 3 diseases are different illness. Because they are all condition that affect how the brain functions their symptoms may overlap.The history , the specific combination of symptoms and exams/ tags will help determine which brain related illness is the most appropriate and likely diagnosis.